Switching power supplies are known. As known, such power supplies have an alternating current ("AC") input coupled to the input of a full-wave diode rectifying bridge. The diode bridge converts the AC into direct current, which is then supplied to the diode bridge output. The direct current, in turn, is arranged to charge an output capacitor. When the output capacitor is charged, a direct current output signal is made available from the output capacitor. The output signal, in turn, is time-switched in sequence between several available loads.
Also as known, such power supplies generally switch their loads through load transformers having multiple windings. Typically the load current will be switched through a first winding of a load transformer, with the resulting current transients through the first winding acting to induce current in a second winding of the load transformer. Typically this second winding will be connected in series with a rectifying diode. The output of the diode, in turn, will be arranged to charge an auxiliary output capacitor. When this auxiliary output capacitor is charged, a direct current auxiliary output signal is made available from the auxiliary output capacitor. This auxiliary output is typically used to supply electronic circuits in the primary side of the power supply.
The output capacitor will initially be uncharged when the power supply is initially turned on, that is, when the power supply is initially connected to the AC input. Thus, when the diode bridge begins supplying output voltage to the capacitor, the uncharged capacitor will act to short-circuit the output voltage. As a result of this short-circuit condition presented to the diode bridge, a substantial amount of current will quickly flow, or "inrush", into the charging capacitor. The volume and time duration of this inrush current will vary in direct proportion to the value of the charging output capacitor. Thus, for large capacitors, the resulting inrush current energy can be substantial.
This inrush current is a problem because, at higher levels of current, the safe operating regions of the bridge diodes may be exceeded and, as a result, the diodes may fail. As a result, power supplies usually are equipped with one or more devices to limit the inrush current to safe operating levels.
With respect to this inrush current problem, prior power supplies have included a current-limiting resistor connected in series between the AC input and the diode bridge. With this arrangement, the resistor limited the maximum inrush current that could flow during a power-up condition.
While solving the inrush current problem during power-on, however, this current-limiting resistor has the corresponding negative effect of creating unwanted power loss during normal operation of the power unit. To solve this further problem, prior power supplies used the following arrangement: first, a triac was connected in parallel with the current-limiting resistor and, second, the triac was arranged to become energized during normal operation of the power unit, thus shorting-out the current-limiting resistor to minimize the resistive power loss.
This prior arrangement, however, has the further problem that additional circuitry is required to energize the triac during the normal operation of the power unit. For example, some prior power supplies have used additional transformer windings or additional optical-coupling devices to activate the triac. Such additional components are costly, thus increasing cost to the power supply unit, resulting in a less cost-effective product.
As a result, it is desirable to provide an improved power supply.